Male sex chromosome losing genes by rapid evolution, study reveals

July 17, 2009
A wallaby is a marsupial. Credit: Kateryna Makova, Penn State

Scientists have long suspected that the sex chromosome that only males carry is deteriorating and could disappear entirely within a few million years, but until now, no one has understood the evolutionary processes that control this chromosome's demise.

Now, a pair of Penn State scientists has discovered that this sex chromosome, the Y chromosome, has evolved at a much more rapid pace than its partner chromosome, the X chromosome, which both males and females carry. This rapid of the Y chromosome has led to a dramatic loss of genes on the Y chromosome at a rate that, if maintained, eventually could lead to the Y chromosome's complete disappearance. The research team, which includes Associate Professor of Biology Kateryna Makova, the team's leader, and National Science Foundation Graduate Research Fellow Melissa Wilson, will publish its results in the 17 July 2009 issue of the journal .

A giraffe is a eutherian mammal. Credit: Kateryna Makova, Penn State

"There are three classes of mammals," said Makova, "egg-laying mammals, like the platypus and the echidna; marsupials, like the opossum and the wallaby; and all other mammals -- called eutherians -- which include humans, dogs, mice, and giraffes. The X and Y chromosomes of marsupials and eutherians evolved from a pair of non-sex chromosomes to become ."

Humans have 23 pairs of chromosomes, which are the structures that hold our DNA, but just one pair of these chromosomes are sex chromosomes, while the others are referred to as non-sex chromosomes. "In eutherian mammals, the sex chromosomes contain an additional region of DNA whereas, in the egg-laying mammals and marsupials, this additional region of DNA is located on the non-sex chromosomes," said Makova. "At first, bits of DNA within this additional region were readily swapped between the X and Y chromosomes, but some time between 80 and 130 million years ago, the region became two completely separate entities that no longer swapped DNA. One of the regions became specifically associated with the X chromosome and the other became specifically associated with the Y chromosome."

By comparing the DNA of the X and Y chromosomes in eutherian mammals to the DNA of the non-sex chromosomes in the opossum and platypus, the team was able to go back in time to the point when the X and Y chromosomes were still swapping DNA, just like the non-sex chromosomes in the opossum and platypus. The scientists then were able to observe how the DNA of the X and Y chromosomes changed over time relative to the DNA of the non-sex chromosomes. "Our research revealed that the Y-specific DNA began to evolve rapidly at the time that the DNA region split into two entities, while the X-specific DNA maintained the same evolutionary rate as the non-sex chromosomes," said Makova.

An echidna is a monotreme. Credit: Kateryna Makova, Penn State

Once the biologists determined that the Y chromosome has been evolving more rapidly and has been losing more genes as a result, they wanted to find out why the Y chromosome has not already disappeared entirely. "Today, the human Y chromosome contains less than 200 genes, while the human X chromosome contains around 1,100 genes," said Wilson. "We know that a few of the genes on the Y chromosome are important, such as the ones involved in the formation of sperm, but we also know that most of the genes were not important for survival because they were lost, which led to the very different numbers of genes we observe between the once-identical X and Y. Although there is evidence that the Y chromosome is still degrading, some of the surviving genes on the Y chromosome may be essential, which can be inferred because these genes have been maintained for so long."

The team then decided to test the hypothesis that some of the genes on the Y chromosome are being maintained because they are essential. The team's approach was to compare the expression and function of genes on the Y chromosome with analogous genes on the X chromosome. "If the genes' expressions and/or functions were different, then it would make sense that the genes on the Y chromosome would be maintained because they are doing something that the genes on the can't do," said Makova. "This hypothesis turned out to be correct."

Although some of the genes on the Y chromosome have been maintained, most of them have died, and the team found evidence that some others are on track to disappear, as well. "Even though some of the genes appear to be important, we still think there is a chance that the Y chromosome eventually could disappear," said Makova. "If this happens, it won't be the end of males. Instead, a new pair of non-sex chromosomes likely will start on the path to becoming sex ."

In the future, the team plans to use its newly generated data to create a computer model that tracks the degeneration of the Y chromosome. The scientists hope to determine how long it will take for the Y chromosome to disappear. They also hope to identify the processes that are most important for degeneration of the .

Source: Pennsylvania State University (news : web)

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1 / 5 (3) Jul 17, 2009
Are these mammals in captivity or are they captured from the wild and tested? What is the difference in genes from a captive mammal to one that lives in its wild habitat?
1 / 5 (1) Jul 17, 2009
I'm finding this very hard to comprehend.

What would be the evolutionary benefit of degrading Y to disaprearance only to create another sex chrom from a non-sex one?
There would have to be a parallel run of both during the transition or how would mammals reproduce? I suppose the generation could be all female, but that makes even less sense.

Just because a chrom is shedding genes doesn't mean its on it way out. Vast amount of genetic material comes from varying levels of disease and genetic remnants of viruses. Some of those are non existent now so the benefit of keeping immunity is again non-existent, therefore this genetic code can be removed.

It makes sense that Y would be more changeable than X due to the huge quantity of sperm produced with replications taking place in millions a day. With 2 X chroms there is redundancy built in so even if one is faulty the chances are halved of any issues arising.
3 / 5 (2) Jul 17, 2009
To an extent I'm with you iknow.

Think about it this way, women are far more prone to certain genetic basis diseases, for example: autoimmune diseases like rheumatoid arthritis, lupis, etc.

The unshed genes in the X chromasome could very well give rise to the disparity.
1.6 / 5 (7) Jul 17, 2009
Men are far more prone to sex-linked genetic diseases then women.
5 / 5 (1) Jul 17, 2009
Men are far more prone to sex-linked genetic diseases then women.

sex-linked genetic diseases would be diseases linked to the sex of the person, so all you're saying is there are more diseases that affect men only, vs diseases that affect women only. That would have zero bearing on my statement. Thanks for the 1 though.

On a revisit, think about it this way, the real flaw in the thought process is the fact that, the x chromasome is copied jsut as often, Men produce both chromasomes, so the difference in meiosis vs mitosis may not have that large a bearing on the end result.
not rated yet Jul 17, 2009
I would say that in order for the male chromosome to disappear completely, there would have to be complimentary changes in the female chromosome such that women would no longer need a man's sperm to fertilize their eggs; they would have to evolve the ability to do it themselves.

Otherwise, the shrinking size of the male chromosome could be linked to the fact that males are the ones who compete over the "right" to reproduce with a particular female and not the other way around. The shrinking size of the Y-chromosome would then be caused by an evolutionary pressure to reduce the complexity of the male genes by eliminating duplicate genes or genes that are obsolete. Thus, as the number of genes shrinks, so does the total size of the Y-chromosome.
5 / 5 (1) Jul 17, 2009
But how would a reduction in the size of the chromasome be advantageous in breeding? If anything it would be counter productive, much like a genetic deviant would have a higher chance of not procreating through lacking intellect, social skill, attractiveness, physical prowess, but in this case, a higher chance of sterility, or incompatibility.
5 / 5 (2) Jul 18, 2009
But how would a reduction in the size of the chromasome be advantageous in breeding?

Its not. It simply isn't disadvantageous in most cases of gene loss. The lost genes were either redundant or not needed in men. Unless a particular gene is needed it is free to change or even be lost. The lack of an advantage is probably why after tens of millions of years there are still genes that can be lost without a loss of viability. If there was an advantage to having a shorter chromosome I suspect that the Y chromosome would have been shortened to the bare minimum already.

The Y chromosome does not need genes that are specific to female physiology so those genes can be lost without problems. Genes that are specific to male physiology have to be conserved. When the change arose allowing for chromosomal specialization with only one sex using the Y and both sexes using the X it was inevitable that the Y chromosome would undergo change at different rate than the X chromosome, since only the Y chromosome has the specialization.

The loss of genes in the Y cannot continue indefinitely, not because the Y is only so long, but because some genes cannot be lost without making the males reproductively non-functional. Either by sterilization or damaged health.

Since the Y chromosome cannot exchange genes with the X, for most of its length, the high rate of change MUST be due to a lack of need for the lost genetic material. Normally having a pair of genes that perform the same functions allows for a HIGHER rate of change because having two copies of a single gene can cover problems that a mutated gene can cause. This allows genetic change without always compromising viability. Thus making for more diversity in sexually reproducing chromosomes.

The Y chromosome is no longer a sexually reproducing chromosome, therefor any changes to it MUST be viable and cannot be covered by a second copy since there is no second copy. This will limit the rate of change in the Y chromosome to much lower levels, similar to non sexual species, which I don't think of as species rather than monoclonal lines of descent. Thus the present of change will eventually taper off to a lower rate than that of the X chromosome after the genes that are not needed for reproductive viability are lost. The Y chromosome can only get so short. After that any loss will result in the individual being selected out, possibly long before birth. This is likely occurring at present and for some time in the past.

not rated yet Jul 23, 2009
Is it possable to check a large number of men to see if there are any Y chromesomes that are larger than 250 or 500 or more genes.
Is it possable to recombine Y chromesomes so that it will reproduce its self to a more complete size.
not rated yet Jul 23, 2009
Is it possable to check a large number of men to see if there are any Y chromesomes that are larger than 250 or 500 or more genes.

I doubt that it would be particularly hard to do a gene count but it would take time money and a large range of volunteers. It would be interesting to see if Australian Aborigines had a significant differences in gene count. The count is likely close since this has been going on far longer than human have existed. I wouldn't be surprised if the information already exists.

The Y chromosome IS complete. If it wasn't there wouldn't be any males. Sure would be nice to have a second copy of the genes responsible for hair. Eventually it should be possible to add in genes that we men feel we could use a second copy of. Sure can't be done yet. I don't think we can even target any particular chromosome much less the Y chromosome. And all the methods we presently have for injecting genes into cells are a tad dubious not counting the ones we already know are more likely to kill than do anything even remotely useful.

Genetic engineering is still at the non-human stage except when dealing with a very few of the many genetic diseases that will kill. In all cases that I have heard of it is still a dangerous experiment.

3.7 / 5 (3) Jul 26, 2009
Rash Human Y Demising?
On The Male Sex Chromosome

A. On Human male sex chromosome, aaagain?

From "Male Sex Chromosome Losing Genes By Rapid Evolution, Study Reveals"

"...we also know that most of the genes were not important for survival because they were lost, which led to the very different numbers of genes we observe between the once-identical X and Y. Although there is evidence that the Y chromosome is still degrading, some of the surviving genes on the Y chromosome may be essential, which can be inferred because these genes have been maintained for so long."

"...found evidence that some others are on track to disappear, as well."

B. From "Spontaneous speciation?"

And from "On The Origin And Tasks Of Brain Cells"

And from "Conservation of Y-linked genes during human evolution revealed by comparative sequencing in chimpanzee"

Chimp's genome has been continuing survival by physiologically adapting to changing environments,
whereas Human's genome continued survival mainly by modifying-controling its environment.

C. AcademEnglish verbiage should be at least scientifically careful

The rate of "losing genes" by an organism is not a constant value of a natural law. It is induced and set mostly by the rate and nature of the change of culture of the organism, which is induced, in turn , by various circumstential factors...

IMO we can feel assured that the human Y is not on an accelerating course to oblivion...

Dov Henis
(Comments from 22nd century)

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